1. Field of the Invention
The invention relates to the art of facsimile scanners, and particularly concerns apparatus for scanning graphic copy to produce corresponding electrical signals for transmission to a suitable graphic copy receiver.
2. Description of the Prior Art
Early scanners used for facsimile transmission have used a cylindrical drum around which the subject copy is wrapped, such as described in U.S. Pat. No. 3,561,846, issued Feb. 9, 1971 to D. O. Kingsland. The copy-carrying drum is rotated and a photoelectric element is moved parallel to the axis of the rotating cylinder. The photoelectric element measures the light reflected from the subject copy along a succession of parallel scanning lines. However, since only separate sheets can be scanned on drum scanners, recent improvements have centered on flatbed facsimile scanners, which are more suitable for scanning pages in books, magazines, material mounted on stiff paper, and the like. For example, in one such scanner, rotating polygonal mirrors comprising individual reflecting surfaces have been employed to scan an optical beam across a page and reflect a spot image onto a stationary detector to provide fast side-to-side scanning. As the mirrors rotate, each mirror surface scans one line of information. Slow end-to-end scanning is obtained by mechanically moving the copy horizontally; see, e.g., U.S. Pat. No. 3,523,160, issued Aug. 4, 1970 to R. Willey. However, in such systems, as the light spot is deflected to the side of the page, it tends to become defocused, that is, blurred, distorted, and/or enlarged, because the distance from the rotating mirror to the page changes. This degradation becomes less severe as the distance from the page to the rotating polygon mirror is increased. However, an increase in the size of the scanner is required, and thus a compromise must be made between a small spot and a compact arrangement.
In other systems, a stationary wide-angle lens is used to focus an entire line or even a complete page onto a detector; see, e.g., U.S. Pat. No. 3,562,426, issued Feb. 9, 1971 to J. Lavergne. However, the distance between the document and the lens is dictated in part by lens geometry, and is typically on the order of 20 cm or more. Furthermore, with a stationary wide-angle lens, it is difficult to illuminate the page in such a manner that white areas at the ends and corners of the page produce the same signal level at the detector as white areas at the center of the page.
The distance between the document and a lens is also dictated by the size of the detector. Compactness of the facsimile scanner can be achieved where the detector has approximately the same dimensions as a scanned line, as disclosed in U.S. Pat. No. 3,512,129, issued May 12, 1970 to E. E. Garfield. There, the detector is a linear array of photocells. However, a linear array of conventional photocells, with typically 500 to 2000 individual cells and associated circuitry, is difficult and costly to produce and maintain and would be of relatively large size.